The present invention relates to a method of transparentizing a cellulosic substrate suitable for use as an envelope or mailer, and more particularly, to a method of transparentizing a portion of a substrate utilizing a transparentizing composition.
A number of approaches to providing a transparent portion in an envelope or the like are known. Various types of envelopes or mailers with transparent windows exist where the window consists of a cut-out opening in the mailer substrate which is covered by a transparent patch. The transparent patch is usually secured over the cut-out opening by means of an adhesive, and may consist of any suitable film of transparent material such as glassine, cellophane, or polymeric materials including polyester, polyethylene, polycarbonate, polystyrene, and polyethylene terephthalate. However, such transparent patch configurations have numerous disadvantages. For example, added thickness caused by such window patches over die-cut window openings can cause feeding problems in printers and uneven stacks.
An alternative to the use of a cut-out opening/transparent patch is to apply a transparentizing material to a predetermined portion of the cellulosic mailer substrate to form a substantially transparent window. Such a method entails the impregnation of the cellulosic mailer substrate with a transparentizing material such that the spaces between the fibers of the substrate are filled by the transparentizing material. In order to make the impregnated portion transparent, the transparentizing material must have a refractive index close to that of cellulose (1.5). Examples of known transparentizing methods and materials are disclosed in U.S. Pat. No. 3,813,261 to Muller, U.S. Pat. No. 4,137,046 to Koike et al., U.S. Pat. No. 4,198,465 to Moore et al., and U.S. Pat. No. 5,418,205 to Mehta.
In order to produce high quality cellulosic mailers on a large, industrial scale by employing a transparentizing material, it is desirable that the transparentizing material be capable of achieving at least three important functions: 1) the ability to produce a transparentized portion which possesses a number of physical and chemical properties; 2) the ability to be converted quickly from a penetrating liquid to a solid after impregnation has occurred; and 3) the ability to quickly penetrate the cellulosic mailer substrate in order to fully impregnate the substrate in the shortest time possible. The drawback to producing mailers in this manner, however, is that most transparentizing materials can perform, at most, only one of the aforementioned functions.
Physically, the transparentized portion of a cellulosic mailer substrate should be strong and flexible (i.e., not brittle) and be receptive to inks. Chemically, the transparentized portion should meet U.S. Postal Service specifications for reflectance (sufficient transparency to read the printing beneath the transparentized portion) and PCR (xe2x80x9cPrint Contrast Ratioxe2x80x9d-sufficient contrast between the printing and background beneath the transparentized portion) and should have sufficient resistance to migration and/or volatilization of the transparentizing material from the place where applied on the mailer substrate such that it does not lose its transparency over time.
While some transparentizing materials may be capable of providing fast penetration rates, they are not capable of producing transparentized portions which possess the desired physical and chemical properties. U.S. Pat. No. 5,076,489 to Steidinger, for example, discloses using either wax or oil as the transparentizing material. Wax produces a brittle transparentized area which is easily marred by physical contact therewith to cause a loss of transparency. In addition, wax is not receptive to inks and therefore cannot be printed upon. Oil tends to migrate and/or volatilize easily, thus resulting in a loss of transparency over time.
In an attempt to overcome these problems, liquid polymerizable transparentizing compositions have been utilized in which the paper substrate is first rendered transparent by impregnating it with the liquid polymerizable transparentizing composition and the transparentizing composition is then cured in situ to solidify the transparentized portion. These polymerizable transparentizing compositions offer several advantages over conventional transparentizing materials in that the end-product is usually strong and flexible and does not lose its transparency over time due to migration or/or volatilization. However, there are problems associated with the use of these polymerizable transparentizing compositions. For example, the rate at which some of the liquid polymerizable transparentizing compositions penetrate a cellulosic substrate is so slow that, after applying the transparentizing composition to the substrate, the substrate must be wound up in a tight roll for a period of time to allow the material to impregnate the substrate. See for example, U.S. Pat. No. 4,416,950 to Muller et al. It is known to include a solvent with the polymerizable transparentizing composition to lower the viscosity thereof and thereby speed the rate of penetration of the transparentizing composition into the cellulosic mailer substrate (see, e.g., U.S. Pat. No. 4,513,056 to Vernois et al). However, the use of solvents with transparentizing materials is undesirable due to the added process machinery required to evaporate the solvent from the substrate surface and to recover the evaporated solvent. While it is known to include water or water-alcohol mixtures with the transparentizing material to increase wetting and thus increase the rate of penetration into the substrate (see U.S. Pat. No. 3,813,261 to Muller et al.), the use of water is typically not considered conducive to high-speed production due to the time associated with removing the water from the cellulosic substrate.
Accordingly, there is still a need in the art for a method of transparentizing a portion of a substrate suitable for use as a mailer or envelope which provides complete penetration at high production speeds used to produce envelopes or mailers.
Those needs are met by the present invention which provides a method for transparentizing a cellulosic substrate in which the transparentizing composition is applied at a temperature of less than about 70xc2x0 F. (21xc2x0 C.) to achieve high loading on the substrate. After subsequent heating, the transparentizing composition penetrates the mailer substrate very quickly and completely, and forms a cured polymeric transparentized portion which possesses the aforementioned physical and chemical properties. In this manner, a very high-quality transparentized portion can be formed on cellulosic mailer substrates in a fast, continuous, in-line process.
According to one aspect of the present invention, a method of transparentizing a cellulosic substrate is provided which includes a) providing a cellulosic substrate having first and second surfaces, b) applying a polymerizable transparentizing composition to at least one portion of the first or second surfaces of the substrate, and c) curing the composition. The transparentizing composition is applied at a temperature of less than about 70xc2x0 F. (21xc2x0 C.). Preferably, prior to application, the transparentizing composition is cooled and its viscosity is temporarily raised to allow more of the composition to be transferred to the substrate (i.e., to increase loading).
The transparentizing composition is preferably applied to the substrate by a flexographic printing apparatus which includes a fountain roller, an anilox roller, a plate cylinder and an impression cylinder, where at least one of the fountain roller, anilox roller, plate cylinder, or impression cylinder is preferably cooled. In another preferred embodiment, the substrate is preferably cooled prior to application of the transparentizing composition.
The transparentizing composition is preferably applied on a corresponding portion of both the first and second surfaces of the substrate. The transparentizing composition preferably comprises an acrylated epoxy oligomer and a blend of acrylate monomers. More preferably, the composition comprises from about 45 to 55% by weight of the acrylated epoxy oligomer, from about 35 to 40% of the blend of acrylate monomers. Where the transparentizing composition is cured by ultraviolet radiation, the composition preferably further includes from about 5 to 10% of a photoinitiator, or blends thereof. Preferably, the transparentizing composition has a viscosity of from about 100 to 2500 cps at room temperature (i.e., 70xc2x0 F. (21xc2x0 C.)), and more preferably, about 600 cps. When the transparentizing composition is applied to the substrate, it preferably has a viscosity of about 1200 cps.
In a preferred embodiment of the invention, a primer composition is applied to a portion of the substrate prior to application of the transparentizing composition. The primer functions as a wetting agent which aids in penetration of the transparentizing composition into the substrate. The primer composition preferably comprises a hydrocarbon resin, vegetable oil, a high molecular weight alcohol, and an aliphatic hydrocarbon solvent. In a preferred embodiment, the primer composition comprises from about 35 to 45% by weight of a hydrocarbon resin, from about 20 to 25% by weight vegetable oil, from about 25 to 35% by weight aliphatic hydrocarbon solvent, and from about 5 to 8% by weight high molecular weight alcohol. The primer composition is preferably applied to a corresponding portion of both the first and second surfaces of the substrate prior to application of the transparentizing composition.
After application of the transparentizing composition and the optional primer to the substrate, the substrate is preferably heated to warm the transparentizing composition. This causes the viscosity of the transparentizing composition to drop rapidly and speeds up the penetration of the composition into the substrate.
The transparentizing composition is then preferably cured by radiation such as electron beam or ultraviolet radiation, preferably ultraviolet radiation. The curing step causes the components in the transparentizing composition to polymerize, resulting in a permanently transparentized portion which will not migrate or volatilize. In a preferred embodiment of the invention, the substrate is cooled after the curing step to lower the temperature of the substrate back to room temperature. In an alternative embodiment, the substrate may be cooled after the heating step described above (prior to curing).
After the curing step, indicia may then be printed on the transparentized portion of the substrate. The resulting cellulosic substrate has at least one transparentized portion and may be used as an envelope or mailer. Preferably, the transparentized portion has a smooth interface between itself and the remainder of the substrate, and the transparentized portion has a thickness which is no greater than the thickness of the remainder of the substrate. By xe2x80x9csmooth interfacexe2x80x9d, it is meant that no loose or sharp edges are present which could get caught in process equipment and cause jams or tears. By xe2x80x9ctransparentizedxe2x80x9d, it is meant that there is sufficient transparency to read printing beneath the transparentized portion of the substrate (reflectance of at least 50% in the red spectrum and at least 45% in the green spectrum), and sufficient contrast between the printing and background portion beneath the transparentized portion to provide a print contrast ratio of at least 30%. The transparentized portion of the substrate preferably exhibits an opacity of from about 40 to 65.
Accordingly, it is a feature of the present invention to provide a method of transparentizing a cellulosic substrate suitable for use as a mailer or envelope which allows high loading of the transparentizing composition onto a substrate and forms a cured polymeric transparentized portion.